We have continued work on the high resolution structure of the bifunctional enzyme complex tryptophan synthase. This complex is a paradigm for the study of intersubunit communication and allosteric activity. We have examined the mutant betaK87T which exists as the external aldimine combined with series and with tryptophan. A series of conformational differences have been observed that provide a basis for understanding the mechanism of action and the communication between the two active sites. Domain III of Pseudomonas aeruginosa exotoxin has been crystallized and the structure determined. This domain has similar active site topology to diphtheria toxin and the toxic activity for both involve the ADP ribosylation of elongation factor II. The enzyme was previously crystallized with NAD which undergoes slow hydrolysis to give AMP and nicotinamide. We have now been able to bind a non- hydrolyzable analog of NAD that permits us to visualize the NAD binding site and have determined the crystal structure of the complex which provides a likely model for the binding of NAD and the probable association with EFII. Enzyme I of the phosphoenolpyruvate: Surgarphosphotransferase system (PTS) is the first in a series of enzymes that couple sugar transport through the membrane to phosphorylation in bacteria. The 30kD N- terminal domain of enzyme 1 of (PTS), has been crystallized and the three-dimensional structure determined. Enzyme I transfers its phosphate to the protein Hpr, and we have modeled the structure of the enzyme complexed to the protein Hpr.